The present invention relates to an apparatus for scanning an original, for copier machines, with a continuously variable reducing-enlargement ratio, and to the associated control circuit.
In copiers with illumination of the original, for scanning thereof, a first carriage which carries a lamp and a first mirror is moved parallel to the surface for exposure of the original, at a constant speed V.sub.c. A second carriage which carries one or more mirrors is moved parallel to the first carriage at a speed V.sub.c /2. The movement of the second carriage is derived from the first carriage by means of a pulley rotatably mounted on the second carriage and a cable having one end anchored to the first carriage, and the other end anchored to a fixed point of the frame structure.
With a reduction ratio of 1:1, as is known, the speed V of the first carriage is equal to the peripheral speed of the drum on which the latent image of the original is formed, while the speed V.sub.c /2 of the second carriage is necessary to maintain constant the length of the optical path of the light rays during the scanning stroke of the first carriage.
When the reducing or enlargment ratio changes, it is necessary to alter the speed of the first carriage by an amount which is proportional to the above-mentioned ratio and it is also necessary to modify the initial position of the second carriage with respect to the first carriage by a predetermined amount which also depends on the geometry of the optical system.
In the known copiers, the position of the second carriage with respect to the first carriage is varied by moving the anchorage point of the control cable, by means of levers and cams. In a copier which is known for example U.S. Pat. No. 3,614,222, the movement of the two carriages is produced by an arrangement formed by levers, cable-type return motion means and variableprofile cams to couple the motion of the drum motor to the first carriage and move it at at most three different speeds corresponding to that number of reduction ratios.
However, such an arrangement is very complicated. In addition, it requires a motor with a power output which is higher than that required for rotating the drum and which is used only for short periods in any copying cycle. In another copier which is known for example from published European patent application EP No. 30282, it has already been proposed that the two carriages which are connected by pulleys and cables should be moved by means of a common electric motor, the movement of which is synchronized with the movement of the drum by means of a microprocessor. However, that arrangement is also complicated and expensive to produce.